This invention relates generally to tools for removing coke from containers such as coking drums used in oil refining, and more particularly to improvements in cutting and drilling nozzle designs for use in a decoking tool.
In conventional petroleum refining operations, crude oil is processed into gasoline, diesel fuel, kerosene, lubricants or the like. It is a common practice to recover heavy residual hydrocarbon byproducts through a thermal cracking process known as delayed coking. In a delayed coker operation, heavy hydrocarbon (oil) is heated to a high temperature (for example, between 900° F. and 1000° F.) in large fired heaters known as a fractionation unit, and then transferred to cylindrical vessels known as coke drums which are as large as 30 feet in diameter and 140 feet in height, and typically configured to operate in pairs. The heated oil releases its hydrocarbon vapors (including, among other things, gas, naphtha and gas oils) to the base of the fractionation unit for processing into useful products, leaving behind, through the combined effect of temperature and retention time, solid petroleum coke. This coke residue must be broken up in order to remove it from the vessel, and is preferably accomplished by using a decoking (or coke cutting) tool in conjunction with a decoking fluid, such as high pressure water.
Such a tool includes a drill bit with both drilling and cutting nozzles. The tool is lowered into the vessel through an opening in the top of the vessel, and the high pressure water supply is introduced into the tool so that it can be selectively routed through either the drilling or cutting nozzles of the bit to act as a fluid jet, depending on the mode of operation. Since high flow rates and pressures (for example, flows of 1000 gallons per minute (gpm) at 3000 to 4000 pounds per square inch (psi)) are typically used for such operations, it is neither practical nor desirable to open drilling and cutting nozzles at the same time. Instead, it has been advantageous to employ diverter valves or other flow control devices to selectively direct the flow to either the cutting nozzles or the drilling nozzles, depending on which part of the decoking operation the tool is in at that time. A couple of examples of decoking tools employing mode-shifting attributes are depicted in U.S. Pat. No. 5,816,505 (for manual mode shifting) and U.S. Pat. No. 6,644,567 (for automated mode shifting); both are commonly owned by the Assignee of the present invention, and also incorporated herein by reference.
Regardless of whether a decoking tool uses mode-shifting features, the relatively large size of the tool, coupled with the generally outward-pointing cutting nozzles, means that it must form a significant radial profile in the bed of coke being cut. A conventional tool is approximately 22 inches in diameter and 35 inches long, while the nozzle assembly dimensions are slightly over 5 inches in length with an outer diameter at the inlet of about 3.75 inches and an outer diameter at the exit of about 1.875 inches. This large size exacerbates the tendency of the tool to get stuck, especially in situations where the bored-out passageway formed in the coke may already be compromised, such as when the coke bed collapses or gets stuck with coke pieces that have been liberated by the force of the decoking fluid emanating from the cutting nozzles. Under such a situation, the tool could get stuck, requiring difficult and time consuming steps to free it.
In addition to large physical dimensions, conventional cutting and drilling nozzles tend to exhibit a larger-than-necessary pressure drop. Much of this stems from an unnecessarily large radial profile caused in the plane of ejection of the decoking fluid at the nozzle tip. The conventional nozzle was relatively long in order to accommodate the large number of drilled flow passages, while the large radial dimension reflects the need for numerous such passages. In addition, various components making up the nozzle are formed as an assembly made up of multiple pieces that may necessitate complex machining and related manufacturing.
It is desirable to create nozzles for a decoking tool that avoid one or more of the shortcomings mentioned above.